Fetch_Feed : How Can I change the Cache Time

I wanted to display an External RSS from mybb forum to my wordpress blog. I found out the function fetch feed (http://codex.wordpress.org/Function_Reference/fetch_feed). According to the codex page this function uses SimplePie and FeedCache functionality to retrieve, parse and cache an external feed. My blog is hosted on a shared server and I am worried about server overload. So I wanted to find out when the cache expires. The SimplePie documentation tells that cache expires after every 60 mins. Now the extrenal feed I want to display does not change more than once or twice a day. So how could I change these 60 mins to 180 mins.

Secondly, the fetch_feed page describes a method to display 5 items from a feed. But mybb forum’s syndication could be queried for number of items like this http://www.example.com/forum/syndication.php?limit=5 so I basically don’t need to check the items. So how do I change the code so that it doesn’t check the number of items in the feed. Below is the Fetch_Feed example from codex.